1. Field of the Invention
The present invention relates to an improved fastener assembly for securing a workpiece to a support. More particularly, the present invention relates to a fastener, washer, and stand-off sleeve assembly for fastening a workpiece, which is subject to plastic deformation, to a support such that the workpiece is reliably held in place over an extended period of time.
Materials such as thermoplastic and thermosetting plastics have come into widespread use. Such materials may be relatively easily formed into complex shapes. They are highly corrosion resistant and often cost less than other materials such as metals like aluminum or steel. However, most common plastic materials are more subject to plastic deformation or cold flow than are metals such as aluminum and steel. Further, cold plastic deformation, creep, or flow of a plastic material often occurs when it is fastened to a support by a conventional fastener. That is, the plastic material deforms or creeps out from between the fastening surfaces of the fastener and support. Therefore, the joint initially formed by the fastener loosens in time since the thickness originally confined between the fastening surfaces of the fastener and the support lessens. Accordingly, it is desireable for a fastener to apply a fixed load to the plastic workpiece which is less than will cause plastic deformation of it yet which is sufficient to hold the plastic workpiece securely in position. It is also desirable that the fastener maintain its tightly secured condition against the base or support structure to which the workpiece is attached.
2. Description of the Prior Art
Various attempts to solve the problems in securing plastic or plastic-like materials to a support structure have been proposed in the past. For example, U.S. Pat. Nos. 4,193,434 (Wagner) and 4,238,165 (Wagner) both disclose preassembled fastener units for clamping plastic workpieces to support structures. These fastener units comprise a preassembled bolt and washer-like device. The washer-like device includes a tubular sleeve having a radially inwardly directed protruberance in the upper region of its inner wall. The upper extremity of the shank of the bolt is unthreaded and includes a mating radially outwardly projecting protuberance that cooperates with the protuberance on the sleeve so that the washer-like structure may be snapped onto the shank of the bolt and be axially retained and free spinning thereon.
The washer-like device further includes a radially extending spring flange at its upper extremity that is conical and has its outermost periphery spaced upwardly from the lowermost extremity of the sleeve by a distance substantially equal to the thickness of the plastic workpiece.
The outside diameter of the sleeve is approximately equal to the inside diameter of a hole through the workpiece at the location where the workpiece is to be fastened to the support. Accordingly, when the bolt is taped into the support with the sleeve received in the hole in the workpiece the major clamping load between the head of the bolt and the support is borne by the sleeve. That is, the sleeve acts as a column or load bearing strut. The workpiece is held in place by the radial flange of the washer-like device. The spring characteristics of this flange may be varied to reduce or increase the spring load on the plastic workpiece as the major clamping load is borne by the sleeve.
While both Wagner patents disclose similar fastener devices, the '165 Wagner patent further specifies that the sleeve have an inside diameter substantially equal to the outside diameter of the crest of threads on the threaded portion of the shank of the bolt plus twice the thickness of the support which is assumed to be sheet metal. This dimensional relationship is alleged to increase the stripping torque, that is, the torque to which the fastener may be tightened without stripping the sheet metal support to which the workpiece is attached.
It has been found that certain practical drawbacks characterize the fastener devices disclosed in both Wagner patents. The washer-like device described and illustrated in both patents as a unitary structure is both relatively difficult and expensive to make. It typically may be drawn from a single piece of sheet metal and, therefore, requires relatively large amounts of material to make. Further, if it is attached or snapped onto the bolt before heat treatment, it is likely to be plasticly deformed during assembly. Therefore, little or no spring back of the metal to its original form will occur. When assembled with the bolt, the inside diameter of the protuberance on the sleeve may be stretched to be large enough relative to the diameter of the protuberance on the shank of the bolt so that the whole washer-like structure can fall off of the bolt. Conversely, if the components are assembled after heat treatment of the washer-like device, the spring back of the washer-like device may not be sufficient to hold the washer on the bolt. Again, therefore, the two parts of the preassembled unit may become disassembled.
It has also been found that, despite the claims of the '165 Wagner patent, the space specified between the interior of the sleeve and the crests of threads on the threaded portion of the shank of the bolt is excessive and causes torque at which the bolt strips the support to decrease not increase. Therefore, poor connection of the workpiece to the support results.